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Backward Piping

In the summer of 2003, we tendered a boiler-retrofit project at one of our large high schools. The project consisted of removing an existing boiler and pump and replacing them with two boilers and primary-secondary pumping, with variable-speed drives on the secondary pumps.

As part of the design process, the consultant calculated that we needed to install only 5,000,000 Btuh of capacity, compared with the existing 8,000,000 Btuh. This was because the school did not become as large as was anticipated in the 1960s.

The mechanical contractor completed the project on time and did a good job. Once the cold weather came, there were a couple of minor problems associated with not being able to get enough heat out of some units. There was one unit at the far end of the school that would not heat adequately at all. Both secondary pumps had to be run at 100 percent to get heat to the building.

An in-house heating technician and I completed a quick run through the building. From what we saw, we were pretty sure we had an air problem. Attempts were made to get the air out, but the problem continued. Basically, we decided we needed to install a centrifugal air separator in the piping leading to the unit at the far end of the school.

We went to the building again to look at this unit. We checked the supply and return temperatures on the DDC against a strap-on probe sensor, just to make sure everything was working correctly. As a bit of light humor, we put our hands on the pipes and agreed that the temperature difference our hands were feeling matched the DDC. But when I looked down and saw the pipe labels, I asked the heating technician why the return was hotter than the supply.

After some more checking, we confirmed the pipes had been connected backward in the boiler room when the boiler was replaced. We had the problem corrected, and there are no more heating problems. The system runs on one pump most of the time. As usual, the simple stuff that we overlook is the cause of the problem.

I am amazed, under the circumstances, that the building ran as well as it did with the backward piping.
Art McDonald P. Eng.
School District No. 73
Kamloops, British Columbia, Canada

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